How to Form a Metal HVAC Round Duct

Ductwork is assembled from both pre-formed and self-formed components.

Some HVAC (Heating, ventilation and air conditioning) ductwork is custom-formed, but must be assembled as part of a system. Many of those systems are designed for round ducts. Straight sections are often made with a system called hound pipe or snap lock, which are sheets with crimped seams that can be rolled and locked together to form a galvanized steel tube. One end of each snap lock section is crimped into a slightly conical form, creating a joint so the pieces can be chained together. The rolled tubes are joined with crimped corners or intersection pieces.

Things You'll Need

• Graph paper
• Measuring tape
• Metal shears
• Self-tapping sheet metal screws
• Foil tape

Instructions

1. Layout your duct on a piece of graph paper so your drawing is to scale. This will tell you what type of corners or "elbows" and intersections you need to get and how long your snap-lock lengths need to be.

2. From the drawing, itemize a component list and a cut list for your straight snap-lock sections.

3. Cut lengths of preformed snap-lock sheets according to your cut list. Use sharp, smooth-bladed metal shears. You can mate a little scrap to the crimped edges where you cut through them, which will reduce crumpling from the force of the shears.

4. Open the flange at the cuts, if needed, where the shears cut through them.

5. Wrap the sheets into a tube and lock the flanges together. Once the flanges catch, press the seam gently, giving the tube a rounder form and helping the flanges seat snugly. You may need to use a putty knife to help the flanges fully seat.

6. Place one end of the snap-lock section over the crimped, slightly conical connector coming from a junction or from the previous section of duct. Snug it in place.

7. Fasten the next crimped elbow or junction by sliding it into the other end of the snap-lock section. This forms a full section of round duct. Continue using this assembly method until the duct is complete. You may use sheet metal or vinyl strapping to suspend ductwork from rafters or joists.

8. Reinforce the connections with self-tapping sheet metal screws wherever necessary. They provide a mechanical connection, but they can also act as an extra hand to hold difficult-to-fit pieces in place while you're assembling the duct.

9. Tape around the joints and along snap-lock seams with foil tape to ensure an air-tight seal.

Tips & Warnings

• Sheet metal edges are sharp. After they've been sheared, they can be even sharper. Wear protective gloves when working with sheet metal.

References

• Alpine Home Repair: Ductwork
• The Skinny: Duct Sealing